1. Field of the Invention
The present invention relates to a stator for an electric machine, in particular an EC motor (electronically commutated motor); the stator has a core composed of lamination plates; and the pole teeth of one or more lamination plates of the core are connected to one another by means of respective bridge pieces. The present invention also relates to a method for manufacturing such a stator.
2. Brief Description of the Prior Art
Electronically controlled electric motors are playing an increasingly important role in a series of practical applications. These are electric motors in which the required periodic switching of coils is no longer executed by means of a commutator, but instead by means of electronic switching devices. The electronic switching devices are also accompanied by a rotor position sensor in order to determine when the stator and rotor for the coils to be switched at the moment are situated in a reactive position that is favorable for the switching or that is required for the switching to occur.
DE 198 42 948 has disclosed an electric motor that has a stator manufactured from a lamination stack, which is shown in FIG. 7. The individual lamination plates 1 in this case have indentations 4 in order to enable adjacent lamination plates 1 to engage with one another. The individual poles 2 are connected to one another by means of a number of bridge pieces 3, each of which is provided with a straight slotted groove 9 that is composed of a number of short openings 10 that are separated from one another by means of respective connecting pieces 11. Because of the linear embodiment of the slotted groove 9 and the multitude of connecting pieces 11, however, this known electric motor could have a relatively high detent engagement moment (if an unbeveled rotor is used) as well as a relatively low output.
DE 102 03 272 A1 has disclosed a stator, which is shown in FIG. 8, for an electric machine that overcomes the disadvantages known from DE 198 42 948 in that the slotted groove 12 is embodied so that it extends inclined at a predetermined angle in relation to a center axis A of the stator. In this case, a continuous slotted groove can be respectively provided between each pair of poles 2 in the stator; the top plate 1 and the bottom plate 1 of the stator lamination stack do not have any slots or else a number of respective slotted grooves can be provided, which are offset from one another and extend parallel to one another in the direction of the center axis A of the stator; one or more plates 1 without slots are respectively situated between the individual slotted grooves.
DE 102 42 404 A1 also describes a stator, shown in FIG. 9, which is embodied in the form of a lamination stack and constitutes part of an electric machine; the lamination stack is comprised of stacked lamination plates 1 of an essentially constant thickness and is embodied in the form of a ring with outwardly protruding pole teeth 2. In order to achieve a simpler production than DE 198 42 948 and DE 102 03 272 A1 with similar magnetic properties, the lamination plates 1, which are embodied the same for the entire stator lamination stack, have a section 13 that is situated between the pole teeth 2 and whose thickness is less than the essentially constant thickness.
All of the above-described stators share the common trait that the magnetic circuit of the stator is divided, i.e. segmented, so that the phase coils can be wound separately from the stator and then placed onto the pole teeth of the stator before final assembly of the magnetic circuit. There are many advantages to this method of stator construction. For example, the phase coils can be wound very precisely, thus achieving a higher filling factor of the gaps between the pole teeth and permitting minimization of the end windings, thus making it possible to reduce the length of the motor and consequently also to reduce costs. After the phase coils have been mounted, the stator core that has been provided with the phase coils can be press-fitted into the external flux return ring or inserted into it by means of a thermal fitting process.
The construction of the star-shaped core comprised of individual plates 1 laminated to one another is very complex. In the case of DE 198 42 948, two different forms of lamination plates 1 must be produced, namely the individual pole tooth 2 and the star, i.e. pole teeth 2 connected to one another by means of bridge pieces 3 with connecting pieces 11. These two forms of lamination plate 1 must be laminated to one another in a precise, predefined sequence in order to produce the core. The individual pole teeth 2 laminated to one another are secured only by means of indentations 4 that engage with one another as a result of which either the pole teeth 2 must have a dovetail joint at the end oriented toward the flux return ring in order to assure their mechanical stability and to fix them in their radial position or else it is necessary for the entire stator core to be encapsulated in an epoxy compound.
Generally, the connecting pieces 11 or the thinner sections 13 of the bridge pieces 3 between the pole teeth short-circuit the magnetic flux of the rotor, thus reducing the overall flux brought together by the phase coils and thereby also reducing the output torque of the motor. These connecting pieces 11 or thinner sections 13 can also lead to a non-sinusoidal waveform of the electromotive force when the motor is under load, which can increase an undulation of the electromagnetic moment. The use of connecting pieces 11 or thinner sections 13 also complicates the calculation of the electromagnetic properties of the motor because, for example, the optimization of the motor through the use of 2D finite element methods cannot be based on any known behavior of the precise modeling of such connecting pieces (or partially closed gap openings).
As indicated above, DE 102 42 404 and DE 102 03 272 partially overcome the above-described disadvantages; DE 102 42 404 simplifies the construction of the star-shaped core. The reduction in undulation of the electromagnetic torque that DE 102 03 272 achieves, however, has the disadvantage of a larger number of different forms of pole teeth 2, which must be manufactured and positioned in relation to one another.